The present invention relates to roll bending machines for bending plates with four shaping rolls, of the type comprising an upper roll for dragging the plate to be bent, a central gripping roll and lateral bending rolls placed below the upper roll, wherein said central and lateral rolls are movably supported towards the upper roll dragging the plate.
In roll bending machines, the strictest working limit is represented by the minimum bend radius which can be obtained on the plate which the machine has to bend.
This limit mainly depends on the diameter of the upper roll of the bending machine as well as on the distance between the axes of the lower rolls which must be predefined at the design stage. The plate does in fact have to be rolled around the upper dragging roll in order to be bent into a tubular shape. As a result the diameter of the upper roll defines a working limit below which the plate cannot be physically bent. On the contrary, in view of the "spring back" of the plate, the minimum working diameter which can be obtained is generally greater than the diameter of the upper roll by a given coefficient.
When designing a bending machine the rolls must likewise be dimensioned with a fully defined diameter which takes into account the loads and stresses which working the plate causes to the rolls themselves. The greater the thickness and the dimensions of the plates to be worked, the higher the stresses and the strains acting on the rolls and, consequently, the greater the resulting diameter of the rolls themselves must be.
Another element which conditions the diameter of the rolls of a bending machine is represented by the flexure which the same rolls undergo due to the forces which they exert during working. In fact, if the flexure of the rolls is too high, albeit without involving risks for the rolls themselves, the result of working would be of poor quality and unacceptable since the bent plate would be deformed or would lack the required cylindricity since, due to the excessive flexure of the rolls, a bend with the so-called "barrel effect" would be obtained.
Therefore the rolls of a bending machine must be designed and dimensioned not only to an extent such as to withstand the high stress forces which act during the bending operations, but also in order not to flex excessively.
The actual length of the bending machine affects the flexure of the rolls and hence the diameter of the rolls themselves. Therefore, in bending machines of considerable length, for the purpose of limiting the flexure of the rolls within acceptable values, the diameter has to be increased by introducing further restrictions to working.
These working limits and restrictions of roll bending machines are accepted unwillingly due to the increasingly felt requirement for bending machines capable of offering the maximum working potential.
It would therefore be desirable to have roll bending machines also capable of working plates of considerable length, and with bend radii which are as small as possible.
The plate bending machines of the type mentioned, currently available, are not able to fulfil these requirements.
For this reason special expedients or bending machines of another kind have to be used, with considerably high working costs and with results whose quality is at times poor. In fact the only possibility currently allowed, besides that of using machines of another kind, is of using very short bending machines for bending a high number of plates which then have to be welded side by side with high production costs and with technical and aesthetic results which are not always acceptable by users.
The object of the present invention is to provide a roll bending machine suitable for solving the problems mentioned previously; more precisely a main object of the present invention is to provide a roll bending machine by means of which it is possible to reduce the diameter and the distances between the axes and the rolls in order to overcome the working limits found with traditional roll bending machines.
A further object of the present invention is to provide a roll bending machine, as specified above, which in addition to being able to withstand high forces and stresses, also allows the diameter of the rolls to be reduced to a minimum, maintaining the flexure of the rolls themselves within acceptable limits.
A still further object of the present invention is to provide a roll bending machine, as related, by means of which it is possible to bend plates with extremely small bend radii, and smaller than those which can be currently obtained with traditionally or standard use bending machines.